1. Field of the Invention
The invention relates generally to a computer implemented process for examination of the QRS complex of an electrocardiograph signal in order to predict the likelihood of arrhythmic tachycardia.
2. Description of the Prior Art
It was early recognized that a diseased state in the myocardium will alter the conduction velocity of electrical energy; however, these late potentials showing up in the QRS signal are very small and require specialized data acquisition techniques in order to be detected and measured. It is not possible to examine such low power signals using conventional ECG equipment and, in general, two techniques for high resolution electrocardiography (HRECG) had been available to look at such signals: these were (1) signal averaging and (2) high gain ECG. Signal averaging relies on the fact that the noise in the signal is random, therefore the addition of a large number of signals of like time frame will result in noise cancellation. The latter technique uses very high gain amplifiers and extensive shielding to reduce the amount of noise in the signal, thereby extracting the signals by brute force. The further refinements in detection circuitry have taken the form of various filtering schemes effective to further delineate the late potential indications.
The basic and earliest disclosure of the present form of heart attack prediction method was made by Berbari et al. entitled "Non-Invasive Detection of Delayed Ventricular Activation Presaging Arrythmias", in Circulation, Vols. 59 and 60, Supp. II, October, 1977, Abstract No. 347, page 111-91. This Abstract was soon followed by a publication in The American Journal of Cardiology, Volume 41, page 697-701, April, 1978, entitled "Recording from the Body Surface of Arrythmogenic Ventricular Activity During the S-T Segment"--Berbari et al. This subject was also extensively investigated in the doctoral thesis of Edward J. Berbari entitled "New Engineering Approaches to Non-Invasive HIS-Purkinje System Recordings" as submitted to the University of Iowa in May of 1980. These prior studies were concerned with signal processing and the manner in which the electrocardiograph (ECG) QRS complex may be best filtered with minimum phase shift thereby to remove all but the usable minute voltage indications which convey the indication of ventricular tachycardia.
Another important early contribution to filtering of the QRS complex of the ECG signal is found in the British patent no. 1,556,512, earlier filed in Japan by the Japanese corporation Fukuda Denshi Co., Ltd. in December 1975. This patent recognized the difficulties in refinement of cardiographic signals and sought better forms of filter for eliminating unwanted fluctuating components. This teaching attempted filtering with analog filters and the attendant problems with phase shift and ringing. The method evolved was termed on-line time reversal filtering wherein the input signal is passed through two analog filters in forward and reverse time order and then combined to produce a signal that is relatively free of phase distortion. This method of filtering is further pursued in specific relation to electrocardiogram processing in a work entitled "Linear Phase Filtering--A New Approach to Distortion-Free Electrocardiograms"--by David Tayler et al., as published in Transactions of IEEE, 1985. A similar teaching is contained in IEEE Transactions on Accoustics, Speech, and Signal Processing of October 1974, Vol. 22, No. 5, page 384, in an article entitled "Two-Pass Recursive Digital Filter with Zero Phase Shift"--by J.J. Kormylo and V.K. Jain. This teaching is directed to elimination of phase distortion by a two-pass scheme where the signal is processed in both the causal and the reverse causal directions.
Yet another pertinent teaching relative to examination of the QRS complex of the ECG is set forth in U.S. Pat. No. 4,422,459 in the name of Simson as issued on Dec. 27, 1983. This patent teaches the filtering of each sample point of the QRS complex signal only once; however, a selected front portion of the QRS complex is filtered in the forward direction while the remaining latter portion of the QRS is filtered in the reverse direction. This is yet another approach to the filtering of the signal averaged QRS complex. This approach was also generally discussed in an article by Simson entitled "Use of Signals in Determining QRS Complex to Identify Patients with Ventricular Tachycardia After Myocardial Infarction" as published in Circulation, Volume 64, No. 2, August 1981 at pages 235-241.